Process and apparatus for separation of nitrogen from lng

ABSTRACT

The present application is concerned with processes and apparatuses for the separation of nitrogen from liquefied natural gas feeds. The processes comprise the steps of: (i) cooling the feed and passing the feed to a fractionation column; (ii) withdrawing from the fractionation column an overhead vapour stream having an enriched nitrogen content, and a liquid stream having a reduced nitrogen content; (iii) dividing the overhead vapour stream from step (ii) into at least first and second overhead streams; (iv) compressing, cooling and at least partially condensing at least the first overhead stream from step (iii); and (v) expanding the stream from step (iv) and passing the expanded stream to the fractionation column as reflux, and wherein cooling in step (iv) is provided, at least in part, by heat exchange with one or more streams from the fractionation column.

This invention relates to processes and apparatus for the separation ofnitrogen from liquefied natural gas (LNG)-mixtures comprising nitrogengas and low-boiling hydrocarbons, such as methane, ethane, propane andbutane.

Nitrogen is found in many natural gas reservoirs, sometimes atrelatively high levels, for example greater than around 5 mol %, whichcan necessitate removal to meet specifications for use as fuel, butoften at lower levels not requiring removal. As high quality gas fieldsare depleted, natural gas increasingly needs to be sourced from lowerquality gas fields, containing higher levels of contaminants such asnitrogen.

Many natural gas reservoirs are not sufficiently close to gas consumersto make pipeline transportation economical and infrastructure has grownworldwide for the transportation of gas in liquefied form as LNG. Thepresence of greater than about 1 mol % nitrogen in LNG, can lead toauto-stratification and rollover in storage tanks, which presents asignificant safety concern, and there is therefore a need for efficienttechniques for the separation of nitrogen from LNG, even for relativelylow nitrogen levels.

For relatively low nitrogen levels of approximately 1 to 2 mol %,nitrogen removal from LNG can be achieved by the separation of thenitrogen rich vapour, also referred to as “flash gas”, which is evolvedwhen the pressure of sub-cooled LNG is reduced to the LNG storage tankpressure—which is typically just above atmospheric pressure. For feedgas nitrogen levels of greater than about 2 mol % of nitrogen, afractionation column is typically employed to achieve separation ofnitrogen from the LNG, while avoiding excessive flash gas flow rates.

Fractionation systems used for the separation of nitrogen from liquefiednatural gas typically incorporate a reboiler to produce strippingvapour, required to reduce the nitrogen level in the LNG product to lessthan 1 mol %.

An example of a conventional apparatus for separation of nitrogen fromLNG is shown in FIG. 1.

A nitrogen-containing LNG feed stream (101) already sub-cooled atelevated pressure is further cooled in a reboil heat exchange system(102). The resultant stream (103) is expanded in a hydraulic expansionturbine (104) to give a two-phase stream (105), which is fed to afractionation column (106).

Liquid (140) from the bottom tray (or packed section) of thefractionation column is partially vaporised in the reboil heat exchangesystem (102), to produce stripping vapour (141) which is fed to thefractionation column, and thereby also providing refrigeration tofurther sub-cool the feed stream (101).

A LNG stream (107) having low nitrogen content is withdrawn from thebottom of the fractionation column, and is reduced in pressure across avalve (108) to give a two-phase stream (109). The two-phase stream (109)is then passed to a vapour-liquid separator (111) to separate a flashgas stream (110) and a low pressure LNG stream (112) for storage. Theflash gas stream (110) is passed to a compressor (134), and a resultingcompressed stream (135) is cooled by a heat exchanger (136) to give afuel gas stream (137).

An overhead vapour stream (113) rich in nitrogen, but with significantmethane content, is withdrawn from the top of the fractionation column(106).

While low temperature fractionation processes, such as that shown inFIG. 1, allow a LNG product having a low nitrogen content to beobtained, the nitrogen vapour overhead from the fractionation columngenerally comprises significant amounts of methane as the columnincorporates no rectification section. The methane-containing overheadvapour is typically used as fuel gas for power generation or to drivecompression equipment.

However, restrictions exist as to the nitrogen content of fuel gas whichmay be used in gas turbines, particularly those derived from aeroengines, which can typically burn gases comprising up to 10 mol %, or upto 15 mol % nitrogen, and sometimes as high as 20 mol % nitrogen.

Alternatively, the methane-containing overhead vapour from thefractionation column is sometimes used as part of a refrigeration cyclein processes that require methane as a refrigerant. It would bepreferable if the methane content of the fractionation overhead vapourcould be substantially eliminated. There is therefore a need in the artfor efficient separation processes that are able to separate mixtures ofnitrogen and LNG to form a natural gas product that is low in nitrogen,and preferably substantially free of nitrogen, and also a nitrogenproduct that is low in hydrocarbons, and preferably substantially freeof hydrocarbons.

One solution to the issue of high methane content of the overhead vapourfrom the fractionation column would be to feed the overhead vapour to aseparate nitrogen rejection unit that is able to produce a nitrogenstream with low methane content suitable for venting to the atmosphereand a methane rich stream suitable for use as fuel gas. In addition tocompression and heat exchange equipment, this would require additionalseparation equipment including one or more vapour/liquid separators andfractionation columns.

The process and apparatus of the present invention avoids the necessityfor a nitrogen rejection unit by producing an overhead vapour streamfrom the LNG fractionation that has a suitable composition (i.e.substantially devoid of hydrocarbons) for venting to the atmosphere.

By recycling a portion of the nitrogen-containing overhead vapour streamfrom the fractionation column it has surprisingly been found that animprovement in separation may be obtained. More specifically, therecycled portion may be used as a nitrogen-rich reflux stream, whichnitrogen-rich reflux stream may be efficiently cooled by heat exchangewith one or more streams withdrawn from the fractionation, particularlyagainst evaporating methane rich liquid streams. By avoiding a separatenitrogen rejection unit, thermodynamic losses are reduced and processefficiency is increased, leading to greater LNG production with lowerpower consumption, as well as improved separation in the fractionationsystem. The process of this invention also avoids the operatingcomplexity of a separate nitrogen rejection unit and is robust tochanges in feed composition.

In accordance with the present invention, there is provided a processfor the separation of nitrogen from a liquid feed comprising liquefiednatural gas and nitrogen, the process comprising the steps of:

-   -   (i) cooling the feed and passing the feed to a fractionation        column;    -   (ii) withdrawing from the fractionation column an overhead        vapour stream having an enriched nitrogen content, and a liquid        stream having a reduced nitrogen content;    -   (iii) dividing the overhead vapour stream from step (ii) into at        least first and second overhead streams;    -   (iv) compressing, cooling and at least partially condensing at        least the first overhead stream from step (iii); and    -   (v) expanding the stream from step (iv) and passing the expanded        stream to the fractionation column as reflux,    -   wherein cooling in step (iv) is provided, at least in part, by        heat exchange with one or more streams from the fractionation        column.

By recycling a portion of the overhead vapour stream to thefractionation as reflux, the process of the present invention adds arectification section to the fractionation column, which enables anoverhead stream to be obtained from the fractionation column that issubstantially devoid of hydrocarbons. For example, the process of thepresent invention is capable of producing an overhead stream from thefractionation column that comprises less than 2 mol % methane, less than1 mol % methane, less than 0.5 mol % methane, and potentially as low as0.1 mol % methane.

Furthermore, by cooling and at least partially condensing the compressedfirst overhead stream in heat exchange with one or more streams from thefractionation column, the heat integration of the process is improved,thereby reducing energy demands.

The proportion of the overhead vapour stream from step (ii) that isrecycled to the column as reflux in steps (iii) to (v) is preferably inthe range of from 20 to 80 mol % of the total overhead vapour stream,more preferably 30 to 70 mol %, and most preferably 40 to 60 mol % ofthe total overhead vapour stream from the fractionation column. However,the exact amount of reflux depends on the nitrogen content of the feedand overhead stream purity. An advantage of the present invention isthat liquid feeds comprising various quantities of nitrogen can beprocessed while maintaining methane content of the overhead streamsimply by varying the proportion of the overhead vapour stream that isrecycled to the column as reflux.

As used herein, the expression “one or more streams from thefractionation column” refers to any liquid or gas from the fractionationcolumn that can be used as a source of refrigeration to cool acompressed overhead stream from step (iv). Thus, the expression mayrefer to overhead vapour withdrawn from the top of the fractionationcolumn. The expression may also refer to the liquid stream withdrawnfrom the bottom of the fractionation column. The expression may furtherrefer to a side stream obtained from an intermediate stage of thecolumn. Still further, the expression may refer to liquid and/or vapourwithin the column where one or more heat exchange steps takes placewithin the column.

The LNG feed may consist of, or consist substantially of, methane. Thefeed may also comprise small amounts of other hydrocarbons such as, forexample, ethane, propane, butane and/or heavier hydrocarbons. Thehydrocarbons in the LNG feed usually comprise greater than 80 mol %methane, more typically greater than 85 mol % methane, and potentiallyup to near 100% methane. The balance of the LNG feed may compriseethane, propane, butane and/or heavier hydrocarbons. Preferably thetotal content of ethane and/or propane and/or heavier hydrocarbons inthe LNG feed is less than 20 mol %, more preferably less than 10 mol %,and most preferably less than 5 mol %. The total content of propaneand/or heavier hydrocarbons in the LNG feed is preferably less than 10mol %, more preferably less than 5 mol %, and most preferably less than2 mol %. The total content of hydrocarbons heavier than propane in theLNG feed is preferably less than 5 mol %, more preferably less than 2mol % and most preferably less than 1 mol %.

The process of the present invention may be used in particular for theseparation of nitrogen from LNG feeds that comprise up to 40 mol %nitrogen. For instance, the feed may comprise up to 30 mol % nitrogen,up to 20 mol % nitrogen or up to 15 mol % nitrogen. Preferably, the feedcomprises at least 1 mol % nitrogen, for example at least 2 mol %nitrogen, possibly 5 mol % nitrogen, or at least 10 mol % nitrogen.

The present invention is particularly applicable to the separation ofnitrogen from feeds that comprise or consist of liquefied natural gas.

The fractionation column is typically operated in a pressure range offrom 100 to 500 kPa, more preferably 100 to 300 kPa, and most preferably120 to 200 kPa. For example, suitable operating pressures for thefractionation column include 130 kPa, 140 kPa, 150 kPa, 160 kPa, 170kPa, 180 kPa and 190 kPa. The operating temperature of the fractionationcolumn is dependent on the operating pressure, but the overheadtemperature is generally in the range −175° C. to −190° C. and thebottom liquid temperature is generally in the range −135° C. to −160° C.

It will be appreciated that, as used herein, expressions such as “anoverhead vapour stream having an enriched nitrogen content” or “a liquidstream having a reduced nitrogen content” are intended to refer to therelative nitrogen content of the respective stream when compared withthe nitrogen content of the feed. Thus, an overhead vapour stream havingan enriched nitrogen content is one that comprises a higher molefraction of nitrogen than that of the feed. Similarly, a liquid streamhaving reduced nitrogen content is one that comprises a lower molefraction of nitrogen than that of the feed.

Generally, the feed is supplied to the fractionation column at or aroundthe operating temperature and pressure of the column. Suitable operatingtemperatures and pressures for the fractionation column are discussedabove. In most cases, the LNG feed will be supplied at a pressuresignificantly higher than the operating pressure of the fractionationcolumn. For example, a liquefied natural gas feed would typically have apressure in the range of from 3,000 to 10,000 kPa. In most cases it istherefore necessary to expand the cooled feed to column pressure. Forexample, the feed may be expanded to column pressure across an expansionvalve or by way of an expansion turbine. Expansion turbines have thebenefit of extracting work from the process under near-isentropicconditions and reducing the amount of upstream cooling of the feed LNGthat is necessary, and thus providing a reduction in energy requirementsin upstream liquefaction plant.

The cooled and expanded feed is preferably supplied to the fractionationcolumn as a two-phase vapour-liquid mixture. More preferably, thetwo-phase mixture has a vapour fraction of from 1 to 40 mol %, morepreferably 2 to 20 mol %, and most preferably from 3 to 10 mol %, forexample 5 to 8 mol %.

In one embodiment, cooling in step (iv) is provided, at least in part,by heat exchange with at least a portion of the overhead vapour streamwithdrawn from the fractionation column.

In a further embodiment, cooling in step (iv) is provided, at least inpart, by heat exchange with at least a portion of the liquid productstream withdrawn from the fractionation column. The portion of theliquid product stream passed in heat exchange with the compressed firstoverhead vapour stream in step (iv) may optionally be expanded first, inorder to provide further cooling.

Heat to the fractionation column is preferably provided, at least inpart by a reboiler. Heated vapour from the reboiler is fed back to thefractionation column to strip nitrogen from the descending LNG in thecolumn.

Where the fractionation column comprises a reboiler, cooling in step(iv) may be provided, at least in part, by heat exchange with a streamfrom the fractionation column in the reboiler.

The type of reboiler that may be used is not limited, and a person ofskill in the art can select suitable reboiler systems. For example,thermosyphon, forced-circulation, and kettle reboilers may be used inthe process of the invention. The fractionation column may comprise aninternal reboiler, located within the column, or an external reboilerlocated outside the fractionation column.

Where an internal reboiler is used, the reboiler is preferably immersedin boiling liquid at the bottom of the fractionation column. Where anexternal reboiler is used, a liquid stream is withdrawn from the columnand fed to the reboiler to produce a heated vapour stream, which isreturned to the column as stripping vapour.

In preferred embodiments, the bulk of the condensing duty required tocool and at least partially condense the compressed overhead stream fromstep (iv) is provided by heat exchange with one or more evaporatingmethane-rich liquids, which may be selected from the liquid productstream withdrawn from the fractionation column and/or boiling liquid atthe bottom of the fractionation column and/or a liquid side-stream thatis withdrawn from the column and fed to a column-side reboiler.

In another embodiment, further cooling of the first overhead stream fromstep (iii) may be provided by heat exchange with the LNG feed.

In another embodiment, heat exchange with a liquid stream from thefractionation column in the reboiler may be used to provide cooling ofthe feed in step (i).

The reflux feed is fed to the top stage of the fractionation column instep (v) to provide rectification of the column vapour, and therebyimprove separation, reducing methane content of the column overheadvapour. The reflux stream is expanded before being fed to thefractionation column, for example using an expansion valve or anexpansion turbine.

The liquid stream withdrawn from the fractionation column in step (ii)is preferably expanded to form a two-phase vapour-liquid stream. Asnoted above, this expanded stream, or a portion thereof, may be used toprovide cooling in step (iv). The two-phase stream is then passed to avapour-liquid separator to separate a low pressure liquid hydrocarbonstream substantially free of nitrogen, and a hydrocarbon vapour stream.Preferably, the hydrocarbon vapour stream contains less than about 15mol % nitrogen, and more preferably less than about 10 mol % nitrogen.The hydrocarbon vapour stream may advantageously be compressed andcooled to form a fuel gas product. Alternatively, this stream may beused as part of a refrigeration cycle.

In accordance with another aspect of the present invention, there isprovided an apparatus for the separation of nitrogen from a feedcomprising liquefied natural gas and nitrogen, the apparatus comprising:

-   -   (i) means for cooling and expanding the feed;    -   (ii) a fractionation column for producing an overhead vapour        stream and a bottom liquid stream;    -   (iii) means for conveying the cooled and expanded feed from        step (i) to the fractionation column;    -   (iv) means for dividing the overhead vapour stream from the        fractionation column into at least first and second overhead        streams;    -   (v) means for compressing at least the first overhead stream;    -   (vi) one or more heat exchangers for cooling and at least        partially condensing the compressed stream from step (v) in heat        exchange with one or more streams from the fractionation column;    -   (vii) means for conveying at least one stream from step (v) to        the one or more heat exchangers; and    -   (viii) means for conveying the compressed, cooled and at least        partially condensed first overhead stream from step (vi) to an        expanding means and from the expanding means to the        fractionation column as reflux.

In a preferred embodiment, the apparatus of the invention comprises aheat exchanger for cooling and condensing at least one stream from step(v) in heat exchange with at least a portion of the overhead vapourstream withdrawn from the column.

In a further preferred embodiment, the apparatus of the inventioncomprises a heat exchanger for cooling and condensing at least onestream from step (v) in heat exchange with at least a portion of thecondensed stream withdrawn from the column. The apparatus may furthercomprise an expander to expand the condensed stream withdrawn from thecolumn, or a portion thereof, before heat exchange with the at least onestream from step (v).

The fractionation column may comprise one or more reboilers, which maybe internal or external to the column. The type of reboiler that may beused is not particularly limited, and thermosiphon, forced-circulation,and kettle reboilers are examples of reboiler types that are compatiblewith the apparatus of the invention.

Where the fractionation column comprises a reboiler, the reboiler may bea heat exchanger as specified in step (vi) which is operable to providecooling and condensing to the at least one stream from step (v) via heatexchange with a stream from the fractionation column.

In a further embodiment, the reboiler is operable to provide cooling tothe feed via heat exchange with a stream from the fractionation column.

The apparatus of the invention preferably comprises means for expandingthe bottom liquid stream from step (ii), at least a portion of which maybe passed to a heat exchanger in step (vi) as described above, avapour-liquid separator, and means for conveying the expanded stream tothe vapour-liquid separator to separate a liquid stream and a vapourstream. The apparatus may further comprise means for compressing andcooling a vapour stream withdrawn from the vapour-liquid separator.

Suitable means for expanding the stream from step (vi) and optionallythe bottom liquid stream from step (ii) include expansion valves andexpansion turbines.

Suitable operating parameters for the process of the present inventionare disclosed in detail above. It is to be understood that the apparatusof the invention is operable in accordance with parameters discussedabove in connection with the process of the invention. Furthermore,preferred operating parameters for the process of the invention are alsopreferred operating parameters for the apparatus of the invention.

The invention will now be described in greater detail with reference topreferred embodiments and with the aid of the accompanying figures, inwhich:

FIG. 1 shows a conventional fractionation apparatus for the separationof nitrogen from a gaseous mixture comprising nitrogen gas andhydrocarbons, as described above;

FIG. 2 shows a fractionation apparatus in accordance with the presentinvention; and

FIG. 3 shows another embodiment of a fractionation apparatus inaccordance with the present invention.

In the embodiment of the invention shown in FIG. 2, a sub-cooled liquidfeed stream (201) comprising LNG and nitrogen is further cooled inreboil heat exchange system (202), to give stream (203). Stream (203) isexpanded in hydraulic expansion turbine (204) to give a two-phase stream(205) which is fed to the fractionation column (206).

A liquid stream (240) is removed from the bottom tray (or packedsection) of the fractionation column below the two-phase feed stream(205) and is partially vaporised to produce stripping vapour (241) inreboil heat exchange system (202), providing refrigeration to furthersub-cool the feed stream (201) and condense nitrogen rich reflux stream(223).

Nitrogen rich overhead vapour (213) is removed from the fractionationcolumn (206) and passed to a heat exchange system (214) where it iswarmed to a suitable temperature (215) for atmospheric venting (217)downstream of a pressure control valve (216).

A portion (218) of the nitrogen rich overhead vapour (213) from thefractionation column is compressed in a compression system (219) to givea compressed stream (220). Compression system (219) includes inter-stagecooling which is not shown. The compressed stream (220) is then cooled(typically in heat exchange against air or water) in a cooler (221) togive a high pressure nitrogen rich stream (222). The high pressurenitrogen rich stream (222) is further cooled in heat exchange systems(214) and (202) to give a liquid stream (224) which is furthersub-cooled in the heat exchange system (214) to provide a sub-cooledliquid stream (225).

The sub-cooled liquid stream (225) is let down to fractionation columnpressure across a valve (226) to give a two phase stream (227) which issupplied as reflux to the fractionation column (206).

A LNG stream (207), with low nitrogen content, is removed from thefractionation column (206), and is let down to storage pressure across avalve (208) to give a two-phase stream (209). The two-phase stream (209)is then passed to a vapour-liquid separator (211) and a flash gas stream(210) and a low pressure LNG product stream (212) for storage areobtained. The flash gas stream (210) is compressed using a compressor(234), giving a stream (235), which is cooled (typically in heatexchange against air or water) in a cooler (236) to give a fuel gasstream (237).

In this embodiment of the invention, the necessary refrigeration to cooland sub-cool the nitrogen rich reflux stream (222) to form the stream(225) is provided by heat exchange with the overhead vapour (213) fromthe fractionation column (206) and the liquid stream (240) that is fedto the reboil heat exchange system (202).

The embodiment of the invention shown in FIG. 3 differs from that ofFIG. 2 by way of the heat exchange systems that are employed to providethe reflux stream to the fractionation column (306). Thus, a sub-cooledliquid feed stream (301) comprising LNG and nitrogen is further cooledin a reboil heat exchange system (302), to give a stream (303). Stream(303) is expanded in hydraulic expansion turbine (304) to give atwo-phase stream (305) which is fed to the fractionation column (306).

A liquid stream (340) is removed from the bottom tray (or packedsection) of the fractionation column (306) below the two-phase feedstream (305) and is partially vaporised to produce stripping vapour(341) in the reboil heat exchange system (302), which providesrefrigeration to further sub-cool the feed stream (301).

Nitrogen rich overhead vapour (313) from the fractionation column (206)passes to a heat exchange system (314) where it is warmed to a suitabletemperature (315) for atmospheric venting (317) downstream of pressurecontrol valve (316).

A portion (318) of the nitrogen rich overhead vapour (313) from thefractionation column is compressed in a compression system (319) to givea compressed stream (320). The compression system (319) includesinter-stage cooling which is not shown. The compressed stream (320) isthen cooled (typically in heat exchange against air or water) in acooler (321) to give a high pressure nitrogen rich stream (322). Thehigh pressure nitrogen rich stream (322) is further cooled andsub-cooled in the heat exchange system (314) to provide a sub-cooledliquid stream (325).

The sub-cooled liquid stream (325) is let down to fractionation columnpressure across a valve (326) to give a two phase stream (327) which issupplied as reflux to the fractionation column (206).

A LNG stream (307), with low nitrogen content, is removed from thefractionation column (306). A portion (328) of the stream (307) is letdown to just above storage pressure across a valve (330) to produce atwo-phase stream (331), which is vaporised to provide refrigeration inthe heat exchange system (314). A remaining portion (329) of the stream(307) is let down to storage pressure across a valve (308) to give atwo-phase stream (333). The stream (332) resulting from vaporisation ofthe stream (331) in the heat exchange system (314) is combined with thestream (333) to give a two-phase stream (309). The two-phase stream(309) is then passed to a vapour-liquid separator (311) to separate aflash gas stream (310) and a low pressure LNG product stream (312) forstorage. The flash gas stream (310) is compressed by means of acompressor (334), giving a stream (335), which is cooled (typically inheat exchange against air or water) in a cooler (336) to give a fuel gasstream (337).

EXAMPLES Example 1 Comparative Example

Table 1 shows operating data for the separation of nitrogen from a LNGfeed comprising 10 mol % nitrogen, 85 mol % methane, 4 mol % ethane and1 mol % propane, at a mass flow rate of 200,000 kg/h, according to theprior art separation system described in FIG. 1. Reference is made tothe vapour fraction, temperature, pressure, mass flow, and molarcomposition of specific numbered streams (numbering of streams as inFIG. 1).

TABLE 1 Stream Number (101) (103) (105) Description LNG Feed ExpanderInlet LNG to Column Vapour Fraction (molar) 0.000 0.000 0.081 VapourLiquid Temperature (° C.) −141.2 −157.1 −166.7 −166.7 −166.7 Pressure(kPa(a)) 5300 5250 170 170 170 Mass Flow (kg/h) 200000 200000 20000021281 178719 Molar Flow Methane (kmole/h) 9402 9402 9402 320 9081Nitrogen (kmole/h) 1106 1106 1106 576 530 Ethane (kmole/h) 442 442 442 0442 Propane (kmole/h) 111 111 111 0 111 Total (kmole/h) 11061 1106111061 897 10164 Stream Number (107) (109) (110) Description ColumnVapour-Liquid Flash Gas Bottom Liquid Separator Inlet Vapour Fraction(molar) 0.000 0.039 Vapour Liquid 1.000 Temperature (° C.) −156.3 −161.7−161.7 −161.7 −161.7 Pressure (kPa(a)) 170 105 105 105 105 Mass Flow(kg/h) 161338 161338 6321 155017 6321 Molar Flow Methane (kmole/h) 87988798 330 8469 330 Nitrogen (kmole/h) 72 72 37 35 37 Ethane (kmole/h) 442442 0 442 0 Propane (kmole/h) 111 111 0 111 0 Total (kmole/h) 9423 9423366 9056 366 Stream Number (112) (113) (135) (137) Description LNGColumn Overhead Compressed Cooled Flash Product Vapour Flash Gas GasVapour Fraction (molar) 0.000 1.000 1.000 1.000 Temperature (° C.)−161.7 −167.0 96.0 40.0 Pressure (kPa(a)) 105 160 5050 5000 Mass Flow(kg/h) 155017 38662 6321 6321 Molar Flow Methane (kmole/h) 8469 604 330330 Nitrogen (kmole/h) 35 1034 37 37 Ethane (kmole/h) 442 0 0 0 Propane(kmole/h) 111 0 0 0 Total (kmole/h) 9056 1638 366 366

It will be noted that the prior art process produces an overhead vapourstream (113) from the fractionation column that comprises a significantamount of methane (37 mol %), and which requires further processing toseparate the remaining nitrogen. This is in contrast with the process ofthe present invention in which the overhead vapour stream (113) issubstantially free of methane (see below).

Example 2

Table 2 shows corresponding operating data for the separation ofnitrogen from the LNG feed used in Example 1, at a mass flow rate of200,000 kg/h, according to the process of the invention as described inFIG. 2.

TABLE 2 Stream Number (201) (203) (205) (207) Description Column LNGExpander Bottom Feed Inlet LNG to Column Liquid Vapour Fraction (molar)0.000 0.000 0.080 Vapour Liquid 0.000 Temperature (° C.) −146.3 −157.1−166.7 −166.7 −166.7 −156.3 Pressure (kPa(a)) 5300 5250 170 170 170 170Mass Flow (kg/h) 200000 200000 200000 21117 178883 171064 Molar FlowMethane (kmole/h) 9402 9402 9402 316 9086 9397 Nitrogen (kmole/h) 11061106 1106 573 533 76 Ethane (kmole/h) 442 442 442 0 442 442 Propane(kmole/h) 111 111 111 0 111 111 Total (kmole/h) 11061 11061 11061 88910172 10026 Stream Number (209) (210) (212) (213) Description ColumnVapour-Liquid Flash Hydrocarbon Overhead Separator Inlet Gas ProductVapour Vapour Fraction (molar) 0.039 Vapour Liquid 1.000 0.000 1.000Temperature (° C.) −161.7 −161.7 −161.7 −161.7 −161.7 −190.9 Pressure(kPa(a)) 105 105 105 105 105 160 Mass Flow (kg/h) 171064 6729 1643366729 164336 67065 Molar Flow Methane (kmole/h) 9397 351 9045 351 9045 12Nitrogen (kmole/h) 76 39 37 39 37 2387 Ethane (kmole/h) 442 0 442 0 4420 Propane (kmole/h) 111 0 111 0 111 0 Total (kmole/h) 10026 390 9636 3909636 2399 Stream Number (217) (218) (222) (223) (224) (225) DescriptionNitrogen Cooled Condensed Sub-cooled Nitrogen Compressor NitrogenNitrogen Nitrogen Nitrogen Vent Suction Recycle Recycle Recycle RecycleVapour Fraction (molar) 1.000 1.000 1.000 1.000 0.000 0.000 Temperature(° C.) 30.0 −73.9 40.0 −154.1 −157.1 −179.0 Pressure (kPa(a)) 101 1202400 2360 2310 2290 Mass Flow (kg/h) 28936 38129 38129 38129 38129 38129Molar Flow Methane (kmole/h) 5 7 7 7 7 7 Nitrogen (kmole/h) 1030 13571357 1357 1357 1357 Ethane (kmole/h) 0 0 0 0 0 0 Propane (kmole/h) 0 0 00 0 0 Total (kmole/h) 1035 1364 1364 1364 1364 1364 Stream Number (227)(235) (237) Description Cooled Nitrogen Recycle Compressed Flash ColumnInlet Flash Gas Gas Vapour Fraction (molar) 0.136 Vapour Liquid 1.0001.000 Temperature (° C.) −191.6 −191.6 −191.6 95.9 40.0 Pressure(kPa(a)) 160 160 160 5050 5000 Mass Flow (kg/h) 38129 5211 32918 67296729 Molar Flow Methane (kmole/h) 7 0 7 351 351 Nitrogen (kmole/h) 1357186 1171 39 39 Ethane (kmole/h) 0 0 0 0 0 Propane (kmole/h) 0 0 0 0 0Total (kmole/h) 1364 186 1178 390 390

It will be noted that the process of the present invention shown in FIG.2 gives rise to a nitrogen rich overhead vapour stream (213) whichcomprises 99.5 mol % of nitrogen, and only 0.5 mol % methane (incomparison with 63 mol % nitrogen and 37 mol % methane in the overheadstream (213) in Example 1). This is reflected in the amount ofhydrocarbon product (212) obtained, which represents 87.1 mol % of thetotal feed, compared with only 81.9 mol % in Example 1.

The improved separation obtained by the process of the present inventioncan be attributed to the provision of the low temperature nitrogen richreflux stream (227), which is obtained at −191.6° C. and supplied to thecolumn at a temperature 25° C. below the feed (205) to the column. Therectification of column vapour provided by this nitrogen rich refluxstream allows the temperature differential between the overhead vapourstream (213) and the liquid hydrocarbon stream (207) to be increased to35.3° C., and the temperature differential between the nitrogen richoverhead vapour stream (213) and the feed (205) to the column to beincreased to 23.3° C., reflective of the increased purity of thenitrogen rich overhead vapour stream (213). In Example 1, in contrast,where no nitrogen rich reflux stream is provided, the temperaturedifferential between the column overhead vapour stream (213) and theliquid hydrocarbon stream (207) is only 10.3° C., and the temperaturedifferential between the column overhead vapour stream (213) and thefeed (205) to the column is negligible at 0.3° C., reflective of muchpoorer separation.

Example 3

Table 3 shows corresponding corresponding operating data for theseparation of nitrogen from the LNG feed used in Example 1, at a massflow rate of 200,000 kg/h, according to the process of the invention asdescribed in FIG. 3.

TABLE 3 Stream Number (201) (203) (205) (307) Description Column LNGExpander Bottom Feed Inlet LNG to Column Liquid Vapour Fraction (molar)0.000 0.000 0.071 Vapour Liquid 0.000 Temperature (° C.) −142.5 −159.0−167.4 −167.4 −167.4 −157.3 Pressure (kPa(a)) 5300 5250 170 170 170 170Mass Flow (kg/h) 200000 200000 200000 18996 181004 172094 Molar FlowMethane (kmole/h) 9402 9402 9402 263 9139 9397 Nitrogen (kmole/h) 11061106 1106 527 579 113 Ethane (kmole/h) 442 442 442 0 442 442 Propane(kmole/h) 111 111 111 0 111 111 Total (kmole/h) 11061 11061 11061 79110270 10063 Stream Number (309) (310) (312) (313) (317) DescriptionHydro- Column Vapour-Liquid Flash carbon Overhead Nitrogen SeparatorInlet Gas product Vapour Vent Vapour Fraction (molar) 0.076 VapourLiquid 1.000 0.000 1.000 1.000 Temperature (° C.) −161.7 −161.7 −161.7−161.7 −161.7 −190.9 30.0 Pressure (kPa(a)) 105 105 105 105 105 160 101Mass Flow (kg/h) 172094 13265 158829 13265 158830 59873 27906 Molar FlowMethane (kmole/h) 9397 692 8705 692 8705 11 5 Nitrogen (kmole/h) 113 7736 77 36 2131 993 Ethane (kmole/h) 442 0 442 0 442 0 0 Propane (kmole/h)111 0 111 0 111 0 0 Total (kmole/h) 10063 769 9294 769 9293 2142 998Stream Number (318) (322) (325) (327) Description Nitrogen Sub-cooledCompressor Nitrogen Nitrogen Suction Recycle Recycle Nitrogen RecycleColumn Inlet Vapour Fraction (molar) 1.000 1.000 0.000 0.120 VapourLiquid Temperature (° C.) −98.4 40.0 −180.5 −191.6 −191.6 −191.6Pressure (kPa(a)) 120 2150 2090 160 160 160 Mass Flow (kg/h) 31968 3196831968 31968 3841 28127 Molar Flow Methane (kmole/h) 6 6 6 6 0 6 Nitrogen(kmole/h) 1138 1138 1138 1138 137 1001 Ethane (kmole/h) 0 0 0 0 0 0Propane (kmole/h) 0 0 0 0 0 0 Total (kmole/h) 1144 1144 1144 1144 1371007 Stream Number (331) (332) (337) Description Cooled LNG RefrigerantSupply LNG Refrigerant Return Flash Gas Vapour Fraction (molar) 0.024Vapour Liquid 0.500 Vapour Liquid 1.000 Temperature (° C.) −160.6 −160.6−160.6 −160.1 −160.1 −160.1 40.0 Pressure (kPa(a)) 125 125 125 105 105105 5000 Mass Flow (kg/h) 15503 396 15107 15503 7390 8113 13265 MolarFlow Methane (kmole/h) 847 18 828 847 443 403 692 Nitrogen (kmole/h) 104 6 10 10 0 77 Ethane (kmole/h) 40 0 40 40 0 40 0 Propane (kmole/h) 10 010 10 0 10 0 Total (kmole/h) 906 22 884 1364 725 725 769

The process of the present invention shown in FIG. 3 also gives rise toan nitrogen rich overhead vapour stream (313) which comprises 99.5 mol %of nitrogen, and only 0.5 mol % methane (in comparison with 63 mol %nitrogen and 37 mol % methane in the overhead stream (313) in Example1).

As in Example 2, the improved separation obtained by the process isattributable to rectification of the column vapour by the lowtemperature nitrogen rich reflux stream (327), which is obtained at−191.6° C. and supplied to the column at a temperature 24.2° C. belowthe feed (205) to the column. This in turn, means that the temperaturedifferential between the nitrogen rich overhead vapour stream (313) andthe liquid hydrocarbon stream (307) is increased to 33.6° C., and thetemperature differential between the nitrogen rich overhead vapourstream (313) and the feed (205) to the column is increased to 23.5° C.,reflecting the increased purity of the nitrogen rich overhead vapourstream.

In this Example, refrigeration to cool the reflux stream (322) isprovided by expanding a portion (328) of the liquid hydrocarbon stream(307). As a result, the flash gas flow is higher in this embodiment at13265 kg/h compared with 6729 kg/h in Example 2, but as the full reboilduty in exchanger (202) is used to sub-cool the feed LNG, the feedtemperature from the liquefaction process can be higher, reducing loadon the upstream liquefaction refrigeration system.

It will additionally be understood that the LNG feed used in the processof the present invention may undergo additional separation and/orconditioning. Examples of such additional processes include one or moreof the following:

-   -   Separation of vapour formed on expansion of the LNG feed stream.        The separated vapour may, in a preferred example, then be        introduced as a separate feed to the fractionation column, and        more preferably to the fractionation column above the main feed;    -   Heating a portion of the LNG feed stream and introducing it as a        separate feed to the fractionation column, preferably above the        main feed. The portion of the stream is preferably heated by way        of a well integrated heat exchange operation;    -   Cooling a portion of the LNG feed stream and introducing it as a        separate feed to the fractionation column, preferably above the        main feed. The portion of the stream is preferably heated by way        of a well integrated heat exchange operation.

It will also be understood that the fractionation column, as describedin the process of the present invention, may additionally comprise orincorporate a side condenser system. In a preferred example, a vapourside draw is taken from an intermediate point, preferably above the mainLNG feed to the fractionation column. In the same way that nitrogen richreflux is generated from the overhead vapour in the embodiments of theinvention described above, the vapour side draw is warmed in a heatexchange operation; compressed and cooled; condensed primarily againstan evaporating liquid stream rich in methane and sub-cooled against coldvapour in a heat exchange operation; expanded to the fractionationcolumn pressure; and returned to the fractionation column as anintermediate two phase feed.

It is believed that the incorporation of a side condenser system reducesthe required overhead reflux flow. As the vapour side draw comprises amixture of methane and nitrogen, it can be condensed against methanerich liquid streams at a lower pressure than can the overhead streamfrom the fractionation column, which is purer in nitrogen. As a largeproportion of the liquid required for rectification can be met by theside condenser system, compression power requirements overall can bereduced.

1. A process for the separation of nitrogen from a liquefied natural gasfeed, the process comprising the steps of: (i) cooling the feed andpassing the feed to a fractionation column; (ii) withdrawing from thefractionation column an overhead vapour stream having an enrichednitrogen content, and a liquid stream having a reduced nitrogen content;(iii) dividing the overhead vapour stream from step (ii) into at leastfirst and second overhead streams; (iv) compressing, cooling and atleast partially condensing at least the first overhead stream from step(iii); and (v) expanding the stream from step (iv) and passing theexpanded stream to the fractionation column as reflux, wherein coolingin step (iv) is provided, at least in part, by heat exchange with one ormore streams from the fractionation column.
 2. A process according toclaim 1, wherein the liquefied natural gas feed comprises from 1 mol %to 40 mol % nitrogen.
 3. A process according to claim 1, wherein coolingin step (iv) is provided, at least in part, by heat exchange with atleast a portion of the overhead vapour stream withdrawn from thefractionation column.
 4. A process according to claim 1, wherein coolingin step (iv) is provided, at least in part, by heat exchange with atleast a portion of the liquid stream withdrawn from the fractionationcolumn.
 5. A process according to claim 4, wherein at least a portion ofthe condensed stream withdrawn from the fractionation column is expandedbefore being passed in heat exchange with the compressed first overheadvapour stream.
 6. A process according to claim 1, wherein heat to thefractionation column is provided, at least in part, by a reboiler.
 7. Aprocess according to claim 6, wherein cooling in step (iv) is provided,at least in part, by heat exchange with a liquid stream from thefractionation column in the reboiler.
 8. A process according to claim 6,wherein the reboiler is an internal reboiler located within thefractionation column.
 9. A process according to claim 6, wherein thereboiler is external to the column.
 10. A process according to claim 9,wherein the feed to the reboiler is a stream withdrawn from the bottomof the fractionation column.
 11. A process according to claim 1, whereinfurther cooling of the first overhead stream from step (iii) is providedby heat exchange with the liquefied natural gas feed.
 12. A processaccording to claim 6, wherein cooling in step (i) is provided, at leastin part, by heat exchange with a liquid stream from the fractionationcolumn in the reboiler.
 13. The process according to claim 1, whereinthe cooled feed in step (i) is expanded to form a two-phase feed to thefractionation column.
 14. A process according to claim 1, wherein thecooled and at least partially condensed first overhead stream from step(iv) is expanded before being fed to the fractionation as reflux in step(v).
 15. A process according to claim 1, wherein the reflux stream ofstep (v) is at a temperature of from 5 to 50° C. below the temperatureof the feed to the column in step (i).
 16. An apparatus for theseparation of nitrogen from a feed comprising liquefied natural gas andnitrogen, the apparatus comprising: (i) means for cooling and expandingthe feed; (ii) a fractionation column for producing an overhead vapourstream and a bottom liquid stream; (iii) means for conveying the cooledand expanded feed from step (i) to the fractionation column; (iv) meansfor dividing the overhead vapour stream from the fractionation columninto at least first and second overhead streams; (v) means forcompressing at least the first overhead stream; (vi) one or more heatexchangers for cooling and at least partially condensing the compressedstream from step (v) in heat exchange with one or more streams from thefractionation column; (vii) means for conveying at least one stream fromstep (v) to the one or more heat exchangers; and (viii) means forconveying the compressed, cooled and at least partially condensed firstoverhead stream from step (vi) to an expanding means and from theexpanding means to the fractionation column as reflux.